Ionizing radiation and especially electron beam accelerators are now employed in a multitude of industrial processes. Crosslinking of polymeric articles and sterilization of medical devices are common place. The use of electron beam accelerators in the sterilization of products such as syringes, surgical gloves, solution bottles, in the medical field is now becoming popular. The popularity corresponds to long-accepted crosslinking applications in the heat shrink cable connector field.
In electron beam sterilization processing, typically, target items will be packaged in a cardboard box type container, a plastic bag or distributed on a flat tray when subject to the beam. The product containing carrier is then placed on a conveyer or appropriate transport means, such as that depicted in U.S. Pat. 4,561,358, and passed through an electron beam chamber. Depending on the particular requirements of the target, the carrier may be subject to dual (multiple) simultaneous source exposure or multiple passes past a single source. These procedures ensure substantially complete, if not uniformly, irradiated articles.
Uniformity problems are often experienced in the processing of non-homogeneous target products, especially in the context of sterilization or irregularly shaped medical products. It is elementary, in the context of sterilization of medical products, that the irradiation procedures must effectively sterilize the entire target but must not adversely affect product functionality. Hence, at the optimal dose, the product is sterilized but is not discolored or degraded in its physical properties. In practice, however, irregularly shaped objects and especially those composed of non-homogeneous material, often present difficulties in achieving uniform irradiation dosage.
Dose uniformity in homogeneous products is represented graphically by a relatively smooth depth-dose distribution curve. Non-homogeneous, or non-uniform products do not lend themselves to such elementary analysis. The problem is particularly acute when a non-homogeneous target is composed of a non-homogeneous material. In such cases the dose (absorbed energy per unit mass) variation within a product can range from a fraction to many times that of the average dose.
There are a number of identifiable factors, independent of the target characteristics, which contribute to the effective dose at a particular location of a product. For example, as a radiation beam is scanned, target portions near the end of the scan present a greater apparent thickness to the beam. The apparent thickness is easily quantified by dividing the product thickness by the cosine of the angle of incidence. As the angle departs from normal incidence, the surface dose increases from enhanced back scattering within the irradiated product, itself. Also, it is elementary that as the (apparent) thickness increases, the effective dose at the back surface of the product is reduced.
Another significant contribution to dose variation, particularly acute in non-homogeneous products, is radiation scattering. When an electron beam enters a product, primary and secondary electrons will scatter at various angles and energies. Since there are fewer primary electrons at the beam scan ends than at the scan center, there are correspondingly far less secondary electrons distributed throughout the thickness of the target. If controlled, electron scatter can be employed to offset the unequal primary dose and can be utilized to promote uniform secondary electron density throughout the product.
One final example of dose contribution is the normal variation at different product depth, itself. Even in a homogeneous product, dose is dependent on the radiation penetration characteristics.